1. Field of the Invention
This invention relates generally to X-ray apparatus, and more specifically to improved apparatus for positioning and centering an X-ray film cassette within an X-ray machine.
2. Description of the Prior Art
Film or other sensitive material to be exposed to X-rays for medical diagnostic and other purposes is commonly packaged in cassettes of varying sizes to suit particular purposes. It is desirable to limit the size of the X-ray beam according to the size of the film cassette. By so limiting the size of the X-ray beam, patient exposure to X-rays is minimized. fIt is also desirable to position the film cassette in the center of the X-ray beam so as to facilitate the design and implementation of apparatus for adjusting the size of the X-ray beam. Additionally, it is desirable to automatically communicate the size of the film cassette to the X-ray beam adjusting apparatus to permit automatic adjustment.
Although adjustment of the X-ray beam can be performed manually by an operator of the X-ray machine, it is preferable to have the X-ray beam automatically adjusted to conform to the size of a particular film cassette. To do so, apparatus must be provided to sense the size of the film cassette. U.S. Pat. No. 3,764,808, issued on Oct. 9, 1973, to R. R. Lackey, et al., disclosed apparatus for this purpose. This device employed a pair of continuously variable transducers, such as rotary potentiometers, mounted on a cassette holder and engageable with a film cassette in order to determine its size. One of the transducers employed a pair of jaws attached to a cable arrangement for centering the film cassette in a first direction, while the other transducer was adjusted by a pivoted arm mounted on the cassette holder and disposed for contacting one side of the film cassette centered between the aforementioned jaws. One problem encountered with this arrangement was that the film cassette might not be centered in the direction perpendicular to the first direction so as to give an erroneous reading to the other of the transducers, resulting in an improper reading of size of the film cassette mounted on the holder.
Another film cassette holder of note is disclosed in applicant's prior filed and copending application Ser. No. 06/318,737, filed Nov. 6, 1981, now U.S. Pat. No. 4,426,724, and entitled "X-Ray Cassette Holder". The film cassette holder disclosed therein acted to center a film cassette in two orthogonal directions using two spring-biased slider-crank mechanisms and acted to measure the size of the film cassette with two linear potentiometers coupled to the slider-crank mechanisms. One disadvantage to this film cassette holder was that to remove the holder and the film cassette from the X-ray machine, the operator needed to pull on the holder with sufficient force to overcome the spring-biased mechanisms. This force was excessive and caused inconvenience in the use of the film cassette holder.